As a power supplying system, a contactless power supplying system which contactlessly supplies power by means of magnetic coupling of paired coils has been conventionally known. The contactless power supplying system is applied more and more to electric vehicles such as electric cars, for example. In the contactless power supplying system, one of the coils, which is connected to an AC power source, is installed in a parking space of a charging station or the like, and the other coil, which is connected to a load (for example, a battery), is installed in an electric vehicle. By using the coil on the parking space side as a transmitting coil (primary coil) and using the coil on the electric vehicle side as a receiving coil (secondary coil), power can be supplied from the AC power source on the parking space side to the battery which is the load on the vehicle side by means of magnetic coupling of the paired coils.
For example, Patent Literature 1 discloses a contactless power transmitting apparatus. This apparatus includes a power transmitting device, a transformer, a power receiving device, and a control device. The transmitting device generates an AC signal of a predetermined frequency which is to be transmitted, and the transformer efficiently transfers power generated in the power transmitting device to the power receiving device side. The control device reduces an oscillation frequency of an oscillation circuit in the power transmitting device to a predetermined value when no load or a small load is connected to the power receiving device. This can reduce the output voltage of the power receiving device and prevent negative effects due to abnormality of the load.